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Big Agnes Lost Dog 50 Sleeping Bag REVIEW

Generously sized summer synthetic top bag that is on the heavy side of lightweight.


by Stuart Bilby | 2006-03-28 03:00:00-07

Big Agnes Lost Dog 50 Sleeping Bag REVIEW


The Big Agnes Lost Dog 50 is a synthetic fill sleeping bag that compresses into a tiny package. It is a top bag with insulation only above the sleeper, relying on a sleeping pad to provide insulation underneath. It is the lightest in the Big Agnes range of top bags at 21.2 ounces. The Lost Dog 50 has 0.4 inches of loft and is rated to 50 degrees F. Not a lot of loft for the weight, but it does have some advantages over ultralight sleeping bags.

What's Good

  • Light weight
  • Compacts remarkably small for a synthetic bag (5 inch diameter x 10 inches long)
  • Good price
  • Seals to a rectangular sleeping pad without gaps at the sides
  • Generously sized - plenty of room to move about
  • Large enough to use as an overbag

What's Not So Good

  • Only 0.4 inch of loft
  • Only 24% of the total bag weight is insulation
  • Poor seal around the neck



2005 Big Agnes Lost Dog 50


Hoodless synthetic fill top bag

  Fill Type

Primaloft Sport 1.8 oz/yd2 (60 g/m2)


Measured loft 0.4 in (0.9 cm), all on top of the sleeper

  Manufacturer Claimed Temperature Rating

50 °F (10 °C)


Measured weight 21.2 oz (600 g); manufacturer's specification 20 oz (567 g)


Regular tested; available in Regular - 72/70/66 in (183/178/168 cm) length/shoulder girth/hip girth, and Long - 78/73/69 in (198/185/175 cm) length/shoulder girth/hip girth


Shell and liner fabric: nylon microfiber ripstop with water-repellent surface treatment (Big Agnes WRM)


6-in (15-cm) baffles sewn through outer shell but not liner, elastic drawcord collar, full length zipper, nylon stuff sack


$109 Regular, $119 Large


The Lost Dog 50 is a lightweight summer top bag. It has about the thinnest sleeping bag insulation available; 0.4-inch thick Primaloft Sport. This leads to the odd situation of only 5 ounces of insulation in a bag that weighs a total of 20 ounces. There are other synthetic fill sleeping bags that weigh the same and have more loft on top, and down sleeping bags that weigh the same with five times the top loft. However, in common with other Big Agnes bags, especially with the Classic Series, the Lost Dog 50 has much more room and is constructed of more durable materials than its ultralight competitors. The generous rectangular shape of the Lost Dog 50 provides ample room and comfort even for larger users. It has a full-length zipper, which adds to the usability - and also to the weight.

As a light summer bag the Lost Dog 50 performs satisfactorily but with one annoying problem. Sealing the gap between your neck and the sleeping pad is often difficult in hoodless top bags and the Lost Dog 50 also struggles here. If the drawcord is pulled tight it strangles your neck and doesn't seal, if it is left loose the gap remains and the drawcord becomes redundant. (A less stiff sleeping pad reduces this effect.) When the temperature dropped a bit and I needed the warmth, the inability to cinch the neck up became a frustration - every time I moved I could feel the warm air pumping out of the bag. For anything but the warmest nights, I needed to wear all the insulated clothes I was carrying to stay warm inside the bag.

Big Agnes Lost Dog 50 Sleeping Bag REVIEW - 1
Pulling the neck drawcord tight puts pressure on your throat and creates a gap for drafts.

Big Agnes Lost Dog 50 Sleeping Bag REVIEW - 2
The Lost Dog 50 seals effectively and securely to a rectangular pad - a feature not as common as it should be in top bags.

While the Lost Dog 50 is large enough to use as an overbag to boost the rating of another bag, the small amount of additional insulation makes it barely worth carrying the extra weight and I found myself looking for something with a bit more loft when I needed additional insulation.

Missing from this review (and for all sleeping bag reviews published here, for that matter) will be an assessment of whether or not the sleeping bag performs adequately at temperatures near its manufacturer-reported temperature rating. Click here for the complete Backpacking Light Position Statement on Sleeping Bag Temperature Ratings.

What's Unique

The Big Agnes Lost Dog 50 is a synthetic top bag with much more girth and a more durable build, but less loft per weight than other down and synthetic summer bags.

Recommendations for Improvement

Doubling the thickness of the Primaloft insulation would only add 25 percent more weight to the Lost Dog 50 and would make it useful in a much wider range of temperatures. Also, a means of sealing around the neck (such as a draft collar attached to the bag bottom) would greatly improve the performance.


"Big Agnes Lost Dog 50 Sleeping Bag REVIEW," by Stuart Bilby. (ISSN 1537-0364)., 2006-03-28 03:00:00-07.